An organic electroluminescent device (hereinafter often called an organic EL device) is a spontaneously luminous device which features higher brightness and higher legibility than those of the liquid crystal devices enabling vivid display to be attained and has, therefore, been vigorously studied.
In 1987, C. W. Tang et al. of Eastman Kodak Company have developed a device of a layer-laminated structure comprising various kinds of materials to bear individual roles, and have put an organic EL device using organic materials into a practical use. The above organic EL device is constituted by laminating layers of a fluorescent body capable of transporting electrons and an organic material capable of transporting holes. Upon injecting both electric charges into the layer of the fluorescent body to emit light, the device is capable of attaining a brightness of as high as 1000 cd/m2 or more with a voltage of not higher than 10 V.
So far, very many improvements have been made to put the organic EL device to practical use. For example, the organic EL device has been widely known having a structure comprising an anode, a hole injection layer, a hole-transporting layer, a luminous layer, an electron-transporting layer, an electron injection layer and a cathode which are arranged in this order on a substrate more finely dividing the roles required for the organic layers between the electrodes than ever before. The device of this kind is achieving a high efficiency and a high durability.
To further improve the luminous efficiency, attempts have been made to utilize triplet excitons and study has been forwarded to utilize a phosphorescent luminous compound.
In the organic EL device, the electric charges injected from the two electrodes recombine together in the luminous layer to emit light. Here, however, what is important is that how efficiently both electric charges of holes and electrons be handed over to the luminous layer. Upon increasing the injection of electrons and their mobility, the holes and electrons recombine together at an increased probability. Further, upon confining the excitons formed in the luminous layer, a high luminous efficiency can be attained. Therefore, the electron-transporting material plays an important role, and it has been urged to provide an electron-transporting material that has a large electron-injection property, a large electron mobility, a high hole-blocking capability and a large durability against the holes.
As for the life of the device, further, the heat resistance and amorphousness of the material also serve as important factors. The material having small heat resistance is subject to be thermally decomposed even at low temperatures due to the heat generated when the device is driven, and is deteriorated. The material having low amorphousness permits the thin film thereof to be crystallized in short periods of time and, therefore, the device to be deteriorated. Therefore, the material to be used must have large heat resistance and good amorphousness.
Tris(8-hydroxyquinoline)aluminum (hereinafter abbreviated as Alq3) which is a representative luminous material has also been generally used as an electron-transporting material having, however, a low electron mobility and a work function of 5.6 eV and, therefore, having a hole-blocking capability which is far from satisfactory.
A method of inserting a hole-blocking layer is one of the measures for preventing the holes from partly passing through the luminous layer to improve the probability of recombination of the electric charge in the luminous layer.
As a hole-blocking material used for forming the hole-blocking layer, for example, there have been known triazole derivatives (see, for example, a patent document 1), bathocuproin (hereinafter abbreviated as BCP) and a mixed ligand complex of aluminum [aluminum(III)bis(2-methyl-8-quinolinato)-4-phenyl phenolate (hereinafter abbreviated as BAlq).
As an electron-transporting material having excellent hole-blocking property, further, there has been proposed a 3-(4-biphenylyl)-4-phenyl-5-(4-t-butylphenyl)-1,2,4-triazole (hereinafter abbreviated as TAZ) (see, for example, a patent document 2).
The TAZ has a work function of as large as 6.6 eV and a large hole-blocking power, and is used for forming an electron-transporting hole-blocking layer that is laminated on the cathode side of a fluorescent luminous layer or a phosphorescent luminous layer prepared by vacuum evaporation or by coating and, therefore, contributes to improving the efficiency of the organic EL devices.
However, a big problem of the TAZ was its low electron-transporting capability. To fabricate an organic EL device, therefore, the TAZ had to be used in combination with an electron-transporting material having a higher electron-transporting capability.
Further, the BCP, too, has a work function of as large as 6.7 eV and a large hole-blocking power but a glass transition point (Tg) of as low as 83° C. In the form of a thin film, therefore, the BCP lacks stability and cannot be said to work as a hole-blocking layer to a sufficient degree.
That is, the above-mentioned materials all lack stability if they are used in the form of a film or are not capable of blocking the holes to a sufficient degree. In order to improve characteristics of the organic EL devices, therefore, it has been desired to provide an organic compound that excels in electron injection/transporting capability and in hole-blocking power, and features high stability in the form of a thin film.